When presented with a suitable antigenic Envelope (Env) structure, the human immune system can produce protective antibodies that HIV is unable to evade. Many of the most potently neutralizing HIV antibodies identified to date preferentially bind the trimeric structure of Env. Thus, recapitulating the native trimeric structure of Env as it exists on the virus and cell surface is a primary objective in developing an immunogen capable of generating broadly protective humoral immunity. The development of a soluble, trimeric Env structure that recapitulates the native, pre-fusion structure of Env has become a major goal for HIV vaccine advancement. However, the inherent instability of the gp120 and gp41subunit complex has posed a major obstacle in designing a soluble Env trimer (gp140) capable of presenting an effective antigenic structure to the immune system. The goal of this project is to engineer a gp140 variant with disulfide bonds that stabilize the trimer to yield conformationally intact, trimeric gp140. To accomplish the Aims of this project, we will engineer and test up to 10,000 gp140 variants that have been systematically mutated to contain paired disulfide bonds designed to covalently stabilize the protein. Each mutant will be individually expressed in human cells to maintain native post-translational processing, and each mutant will be tested for retention of its pre-fusion conformation, antigenic integrity, and trimeric structure.